This theorem states that in any linear bi-lateral network , the current I through any branch of the network due to a single voltage source E elsewhere in the network is equal to the current through the branch of the network where the source was originally placed if the source is placed in the branch in which the current I was originally determined.
APPLICATION OF RECIPROCITY THEOREM:
In fig.(a) the source is shown and the current through resistor r4 is I1. In fig.(b) , the source is placed in series with r4 and the current through r1 is I2. By reciprocity theorem I1 = I2.
The main limitation of this theorem is that it is applicable only to single source networks and not to multi-source networks. Another point to note is that the network should not have any time-varying element.